The present invention relates to a developer apparatus for electrophotographic printing. More specifically, the invention relates to a donor roll, for example, as part of a scavengeless development process.
In the well-known process of electrophotographic printing, a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as toner. Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member, such as paper, and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original or printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways. In the process of electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as development. The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image at a controlled rate so that the toner particles effectively adhere electrostatically to the charged areas on the latent image. A commonly used technique for development is the use of a two-component developer material, which comprises, in addition to the toner particles which are intended to adhere to the photoreceptor, a quantity of magnetic carrier beads. The toner particles adhere triboelectrically to the relatively large carrier beads, which are typically made of steel. When the developer material is placed in a magnetic field, the carrier beads with the toner particles thereon form what is known as a magnetic brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush. This magnetic brush is typically created by means of a developer roll. The developer roll is typically in the form of a cylindrical sleeve rotating around a fixed assembly of permanent magnets. The carrier beads form chains extending from the surface of the developer roll, and the toner particles are electrostatically attracted to the chains of carrier beads. When the magnetic brush is introduced into a development zone adjacent the electrostatic latent image on a photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the carrier beads and onto the photoreceptor. Another known development technique involves a single-component developer, that is, a developer which consists entirely of toner. In a common type of single-component system, each toner particle has both an electrostatic charge, to enable the particles to adhere to the photoreceptor, and magnetic properties, to allow the particles to be magnetically conveyed to the photoreceptor. Instead of using magnetic carrier beads to form a magnetic brush, the magnetized toner particles are caused to adhere directly to a developer roll. In the development zone adjacent the electrostatic latent image on a photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be attracted from the developer roll to the photoreceptor. An important variation to the general principle of development is the concept of "scavengeless" development. The purpose and function of scavengeless development are described more fully in, for example, U.S. Pat. No. 4,868,600 to Hays et al.; U.S. Pat. No. 4,984,019 to Folkins; U.S. Pat. No. 5,010,367 to Hays; or U.S. Pat. No. 5,063,875 to Folkins et al. In a scavengeless development system, toner is detached from the donor roll by applying AC electric field to self-spaced electrode structures, commonly in the form of wires positioned in the nip between a donor roll and photoreceptor. This forms a toner powder cloud in the nip and the latent image attracts toner from the powder cloud thereto. Because there is no physical contact between the development apparatus and the photoreceptor, scavengeless development is useful for devices in which different types of toner are supplied onto the same photoreceptor such as in "tri-level"; "recharge, expose and develop"; "highlight"; or "image-on-image" color xerography. A typical "hybrid" scavengeless development apparatus includes, within a developer housing, a transport roll, a donor roll, and an electrode structure. The transport roll advances carrier and toner to a loading zone adjacent the donor roll. The transport roll is electrically biased relative to the donor roll, so that the toner is attracted from the carrier to the donor roll. The donor roll advances toner from the loading zone to the development zone adjacent the photoreceptor. In the development zone, that is the nip between the donor roll and the photoreceptor, are the wires forming the electrode structure. During development of the latent image on the photoreceptor, the electrode wires are AC-biased relative to the donor roll to detach toner therefrom so as to form a toner powder cloud in the gap between the donor roll and the photoreceptor. The latent image on the photoreceptor attracts toner particles from the powder cloud forming a toner powder image thereon. Another variation on scavengeless development uses a single-component developer material. In a single component scavengeless development, the donor roll and the electrode structure create a toner powder cloud in the same manner as the above-described scavengeless development, but instead of using carrier and toner, only toner is used. In any type of scavengeless development apparatus, one of the most important elements is the donor roll which conveys toner particles to the wires forming the electrode structure in the nip between the donor roll and the photoreceptor. Broadly speaking, a donor roll can be defined as any roll having only toner particles adhering to the surface thereof. To function commercially in scavengeless development, a donor roll should meet certain requirements. In general, a donor roll should include a conductive core and define a partially conductive surface, so that the toner particles may adhere electrostatically to the surface in a reasonably controllable fashion. In hybrid scavengeless development, the donor roll provides an electrostatic intermediate between the photoreceptor and the transport roll. The provision of this intermediate and the scavengeless nip minimizes unwanted interactions between the development system and the photoreceptor, in particular with a pre-developed latent image already on the photoreceptor, before the latent image in question is developed. Minimized interactions make scavengeless development preferable when a single photoreceptor is developed several times in a single process, as in color or highlight color xerography. The donor roll must further have desirable wear properties so the surface thereof will not be readily abraded by adjacent surfaces within the apparatus, such as the magnetic brush of a transport roll. Further, the surface of the donor roll should be without anomalies such as pin holes, which holes may be created in the course of the manufacturing process for the donor roll. The reason that such small surface imperfections must be avoided is that any such imperfections, whether pinholes created in the manufacturing process or abrasions made in the course of use, can result in electrostatic "hot spots" caused by arcing in the vicinity of such structural imperfections. Ultimately, the most important requirement of the donor roll can be summarized by the phrase "uniform conductivity;" the surface of the donor roll must be partially conductive relative to a more conductive core, and this partial conductivity on the surface should be uniform through the entire circumferential surface area. Other physical properties of the donor roll, such as the mechanical adhesion of toner particles are also important, but are generally not as quantifiable in designing a development apparatus. In addition, the range of conductivity for the service of a donor roll should be well chosen to maximize the efficiency of a donor roll in view of any number of designed parameters, such as energy consumption, mechanical control and the discharge time-constant of the surface.